, plus the fact that since tanks are having more bonus armor than before, in the end you'll be negating more armor with less % pen.
Technically true, but in a practical sense its meaningless once you consider how armor works. First of all, the items only gained 20 armor overall. So a level 18 Mao'Kai with just Deadman's and Sunfire (the only 2 armor items to gain armor) for armor before had 200 armor, now he has 220. At 45% bonus armor pen against 200, you'd penetrate 45 armor (as they gave 100 armor, .45 x 100 = 45). At 35% against 220 (the new stats) you're penetrating 42 armor.
"But wait, what about a 3 armor scenario?" Let's use Randuin's, which adds 50. 150 bonus armor x 45% reduction = 67.5 armor reduced, while 170 bonus armor x 35% reduction = 59.5
Now that we've concluded that less armor pen is less armor pen (because a 20 armor difference isn't big enough to make a difference), let's use hypothetical HUGE numbers to showcase a point. Lets say you used to end a build at 300 (200 bonus) armor. The adc builds last whisper, giving 45% bonus pen. 90 armor reduced. Lets say the armor changes gave another 100 bonus armor (total 300 bonus), and last whisper gives 35%. That's 105 armor reduced. That's more armor penetrated. But look at the end- Before you used to end with 110 armor, or 52% damage reduction. Now you end with 290 armor, or 75% damage reduction.